Method for recovery of natural gas liquids for liquefied natural gas

ABSTRACT

The invention includes a process and apparatus for separating a liquefied natural gas (LNG) stream containing methane and lighter components and heavy hydrocarbon components into a more volatile gas fraction containing a substantial amount of the methane and lighter components and a less volatile fraction containing a large portion of the heavy hydrocarbon components. The process includes splitting the LNG stream, preheating and providing to a fractionation tower at two locations. A tower reflux stream is utilized.

Related Applications

This patent application claims priority to U.S. Provisional PatentApplication Ser. No. 60/637,353, filed on Dec. 17, 2004, which isincorporated by reference in its entirety. Also claiming priority toU.S. applications of the same title filed on the same date as thecurrent application.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to the recovery of natural gas liquids(NGL) from liquefied natural gas (LNG) streams.

2. Description of the Related Art

Many attempts have been made to recover NGL from LNG streams. One suchexample can be found in U.S. Pat. No. 6,564,579 issued to McCartneytitled Method for Vaporizing and Recovery of Natural Gas Liquids fromLiquefied Natural Gas Streams (hereinafter “McCartney”). McCartneydescribes a process, as shown in FIG. 4 of the present application, thatuses of an overhead compressor to increase the bubble point of a leangas stream exiting a fractionation tower 10. In this prior art process,a rich low pressure LNG stream 1 is pumped in pump 2 to 450 psia. LNGstream 3 exiting pump 2 is heated in exchanger 5 to −132.5° F. toproduce a heated LNG stream 6. Heated LNG stream 6 is introduced tofractionation tower 10 as a top tower feed stream.

Fractionation tower 10 is a distillation tower that produces a towerbottoms stream 12 and a tower overhead stream 14. Tower bottoms stream12 is sent to a tower reboiler 13 that is used to maintain an amount ofmethane in an NGL product stream 12. A vapor return stream from reboiler13 is sent to fractionation tower 10, while NGL product is drawn off asNGL product stream 12. Tower overhead stream 14 is compressed in leangas compressor 27 to 500 psia to produce a partially boosted stream 28.Partially boosted stream 28 is cooled to −137° F. and completelycondensed in first exchanger 5 to produce a low pressure lean LNG stream21. Low pressure lean LNG stream 21 is then pumped by pump 22 to elevateits pressure to 1422 psia to produce a high pressure lean LNG productstream 23, which is the high pressure lean LNG product that is sent forvaporization and/or energy recovery.

As can be seen in Table II, recovery of LNG products is limited in theprior art. Simulation results of the embodiments of the currentinvention compared to alternate schemes show that for lower recoveriesand NGL production, there is an increase in capital cost due to theaddition of a compressor and increased heat exchanger area for priorart. Utility consumption is also affected.

Co-pending U.S. patent application Ser. No. 10/651,178 titled OptimizedHeating Value in Natural Gas Liquids Recovery Scheme illustrates anotherattempt at recovering NGL products from LNG streams. FIG. 2 illustratesone scheme that uses subcooled rich LNG to increase NGL recovery. Inthis related process, rich low pressure LNG stream 1 is pumped in pump 2to about 525 psia to produce LNG stream 3. LNG stream 3 is then splitinto two streams, a first feed stream 4 and a second feed stream 24.First feed stream 4, which is the larger of the two streams and containsabout 93% of LNG stream 3, is heated in first exchanger 5 to −132.5° F.to produce a heated first feed stream 6. Heated first feed stream 6,which is still all liquid, is introduced to fractionation tower 10 as atower bottom feed stream. Second feed stream 24 is sent as a top feedstream to fractionation tower 10.

As in the prior art process shown in FIG. 4, fractionation tower 10 is adistillation tower that produces a tower overhead stream 14 and a towerbottom stream 12. Tower bottom stream 12 is sent to a tower reboiler 13that is used to maintain an amount of methane in NGL product stream 12.Reboiler vapor stream returns to fractionation tower 10, while NGLproduct is drawn off of reboiler 13 as NGL product stream 12.

Tower overhead stream 14 is cooled to −133.7° F. and completelycondensed in first exchanger 5 to produce low pressure lean LNG stream21. Low pressure lean LNG stream 21 is pumped in pump 22 to elevate itspressure to 1422 psia to produce high pressure lean LNG product stream23, which is the high pressure lean LNG product that is sent forvaporization and/or energy recovery.

SUMMARY OF THE INVENTION

The present invention includes a process and apparatus to increase therecovery of NGL. As an embodiment of the present invention, a process isprovided for separating a liquefied natural gas (LNG) stream into a morevolatile gas fraction containing a substantial amount of methane andlighter components and a less volatile fraction containing a largeportion of heavy hydrocarbon components is advantageously provided. Inthis embodiment, the LNG stream is split into a first feed stream and asecond feed stream. At least a portion of the first feed stream ispreheated and supplied to a fractionation tower as a first tower feedstream.

In preferred embodiments of the present invention, fractionation towerproduces a tower overhead stream and a tower bottoms stream. Toweroverhead stream preferably includes the more volatile fraction of theLNG stream that contains a substantial amount of methane and lightercomponents. Tower bottoms stream preferably includes the less volatilefraction of the LNG stream that contains the heavy hydrocarboncomponents.

At least a portion of the second feed stream is heated and supplied tothe fractionation tower as a second tower feed stream. At least aportion of the tower overhead stream is cooled and partially condensedto produce a partially condensed tower overhead stream. Partiallycondensed tower overhead stream is separated into a lean vapor streamand a tower reflux stream being sent to the fractionation tower. Leanvapor stream is subcooled so that the lean vapor stream is substantiallycondensed thereby producing a lean LNG stream. Lean LNG stream is thenpumped to a high pressure.

In another embodiment of the invention, the liquefied natural gas (LNG)stream is split into a first feed stream and a second feed stream. Thefeed streams define, respectively, a first feed stream enthalpy and asecond feed stream enthalpy. At least a portion of the first feed streamis preheated and provided to the fractionation tower as the first towerfeed stream. From the fractionation tower, the tower overhead stream isproduced containing the more volatile fraction. The tower bottoms streamcontaining the less volatile fraction is also produced from thefractionation tower. The second feed stream is supplied to thefractionation tower such that the first feed stream and the second towerfeed stream have a substantially common composition but the first towerfeed stream enthalpy differs from the second feed stream enthalpy. Thetower overhead stream is compressed to produce a compressed overheadstream. The compressed overhead stream is cooled until it is at leastsubstantially condensed thereby producing a lean LNG stream. In onepreferred embodiment, the compressed overhead stream is cooled until itis subcooled. The lean LNG stream is pumped to a higher pressure therebycreating a high pressure lean LNG stream.

In yet another embodiment of the invention, the process for separatingthe liquefied natural (LNG) stream includes preheating at least aportion of the LNG stream to produce first tower feed stream. The firsttower feed stream is supplied to the fractionation tower that producesthe tower overhead stream and tower bottoms stream. At least a portionof the tower overhead stream is cooled so that the portion of the toweroverhead stream is at least substantially condensed thereby producingthe lean LNG stream. The tower overhead stream can be subcooled. Thelean LNG stream is pumped to a higher pressure to produce a highpressure lean LNG stream. The high pressure lean LNG stream is splitinto a lean tower reflux stream and a lean LNG product stream. The leantower reflux stream is cooled to produce a cooled lean tower refluxstream and the cooled lean tower reflux stream is supplied to thefractionation tower.

Yet another preferred embodiment includes a process for separating theliquefied natural gas (LNG) stream including splitting the LNG streaminto first feed stream and second feed stream. The second feed stream issupplied to the fractionation tower as second tower feed stream toproduce the tower overhead stream and the tower bottoms stream 12including the less volatile fraction containing the heavy hydrocarboncomponents. At least a portion of the first feed stream is preheated toproduce the first tower feed stream. The first tower feed stream issupplied to the fractionation tower. The tower overhead stream iscompressed to produce compressed overhead stream. The compressedoverhead stream is cooled such that the compressed overhead stream issubstantially condensed thereby producing lean LNG stream. The lean LNGstream is pumped to a higher pressure to produce high pressure lean LNGstream. The high pressure lean LNG stream is split into lean towerreflux stream and lean LNG product stream. The lean tower reflux streamis cooled to produce cooled lean tower reflux stream that is supplied tothe fractionation tower.

Yet another embodiment of the invention includes a process forseparating a liquefied natural gas (LNG) stream including preheating atleast a portion of the LNG stream and supplying the at least a portionof the LNG stream to fractionation tower as first tower feed stream. Thefractionation tower produces tower overhead stream and tower bottomsstream. At least a portion of the tower overhead stream is expanded to alower pressure such that the at least a portion of the tower overheadstream is partially condensed to produce a partially condensed lowpressure vapor stream. The partially condensed low pressure vapor streamis separated into a lean vapor stream and tower reflux stream 18. Thelean vapor stream is compressed and cooled to create lean LNG stream.The tower reflux stream is cooled thereby producing a cooled lean towerreflux stream and the lean LNG stream is pumped to a higher pressurecreating high pressure lean LNG stream.

In yet another embodiment of the present invention, the process forseparating a liquefied natural gas (LNG) stream includes preheating atleast a portion of the LNG stream to produce first tower feed andsupplying the first tower feed to fractionation tower. From thefractionation tower is produced tower overhead stream and tower bottomsstream. At least a portion of the tower overhead stream is cooled andthereby partially condensing to produce partially condensed toweroverhead stream. The partially condensed tower overhead stream isseparated into lean vapor stream and lean LNG stream. The lean LNGstream is pumped to a high pressure to produce high pressure lean LNGstream. The high pressure lean LNG stream is split into a lean towerreflux stream 25 and lean LNG product stream. The lean tower refluxstream is cooled to produce cooled lean tower reflux stream. The cooledlean tower reflux stream is supplied to the fractionation tower. Thelean vapor stream is compressed to high pressure to produce compressedsecond lean vapor stream. The compressed second lean vapor stream iscombined with the lean LNG product stream.

In another preferred embodiment, the invention includes an apparatus forseparating a liquefied natural gas (LNG) stream, the apparatus includingmeans for splitting the LNG stream into a first feed stream and a secondfeed stream. Also included is a first exchanger operable to preheat thefirst feed stream to provide a first tower feed stream. A fractionationtower is included for receiving the first tower feed stream and toproduce tower overhead stream and tower bottoms stream. A first cooleris provided that is operable to cool and partially condense at least aportion of the tower overhead stream to produce a partially condensedtower overhead stream. The first cooler is operable to allow for heatexchange between the portion of the tower overhead stream and the secondfeed stream. A separator is operable to separate the partially condensedoverhead tower stream into lean vapor stream and tower reflux stream,the tower reflux stream returning to the fractionation tower. The firstexchanger is operable to allow for heat exchange between the lean vaporstream and the first feed stream, the lean vapor stream being cooled inthe first exchanger to provide lean LNG stream. A pump operable forpumping the lean LNG stream to high pressure LNG stream is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the features, advantages and objects of theinvention, as well as others which will become apparent, may beunderstood in more detail, more particular description of the inventionbriefly summarized above may be had by reference to the embodimentthereof that is illustrated in the appended drawings, which form a partof this specification. It is to be noted, however, that the drawingsillustrate only a preferred embodiment of the invention and aretherefore not to be considered limiting of the invention's scope as itmay admit to other equally effective embodiments.

FIG. 1 is a simplified flow diagram of a hydrocarbon recovery processthat is configured for increased recovery of heavy components from aninlet LNG stream through the use of a split feed stream and lean towerreflux stream in accordance with an embodiment of the present invention;

FIG. 2 is a simplified flow diagram of a hydrocarbon recovery processthat is configured for increased recovery of heavy components from aninlet LNG stream through the use of a split feed stream in accordancewith an embodiment of a process described in co-pending U.S. patentapplication Ser. No. 10/651,178 titled Optimized Heating Value inNatural Gas Liquids Recovery Scheme;

FIG. 3 is a simplified flow diagram of a hydrocarbon recovery processthat is configured for increased recovery of heavy components from aninlet LNG stream through the use of a lean LNG reflux stream inaccordance with an embodiment of the present invention;

FIG. 4 is a simplified flow diagram of a prior art process as shown inU.S. Pat. No. 6,564,579 issued to McCartney, which illustrates a typicalsystem and process for vaporizing LNG and separating natural gas liquidsfrom the LNG stream in accordance with prior art;

FIG. 5 is a simplified flow diagram of a hydrocarbon recovery processthat is configured for increased recovery of heavy components from aninlet LNG stream through the use of an overhead compressor in accordancewith an embodiment of the present invention.

FIG. 6 is a simplified flow diagram of a hydrocarbon recovery processthat is configured for increased recovery of heavy components from aninlet LNG stream through the use of an overhead compressor and a leanLNG reflux stream in accordance with an embodiment of the presentinvention.

FIG. 7 is a simplified flow diagram of a hydrocarbon recovery processthat is configured for increased recovery of heavy components from aninlet LNG stream through the use of an overhead condenser and anoverhead compressor in accordance with an embodiment of the presentinvention.

FIG. 8 is a simplified flow diagram of a hydrocarbon recovery processthat is configured for increased recovery of heavy components from aninlet LNG stream through the use of an overhead expander reflux streamin accordance with an embodiment of the present invention.

FIG. 9 is a simplified flow diagram of a hydrocarbon recovery processthat is configured for increased recovery of heavy components from aninlet LNG stream through the use of a partial flow compressor inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In the description of the figures, the same numbers are used to refer tosimilar or same components in all figures. For clarity and simplicity,not all equipment is shown as they are considered known to those skilledin the art. For consistency, the same equipment parameters such asnumber of trays, efficiencies, pressure drops, product specificationsetc have been used to compare processes.

Embodiments of the present invention have been compared with prior artprocesses to illustrate improvements in either yield or powerconsumptions. The processes described herein were compared using theinlet LNG stream composition and conditions shown in Table I. TABLE IComponent Units Nitrogen Mol % 0.46 Methane Mol % 89.79 Ethane Mol %6.47 Propane Mol % 2.23 i-Butane Mol % .42 n-Butane Mol % .63Temperature F. −241.6 Pressure Psia 71.6 Flow MMTPA 5

In all embodiments of the process, liquefied natural gas (LNG) stream 3contains methane and lighter components and heavy hydrocarboncomponents. In order to recover NGL products, LNG stream 3 is separatedinto a more volatile gas fraction that contains a substantial amount ofthe methane and lighter components and a less volatile gas fraction thatcontains a large potion of the heavy hydrocarbon components.

Referring to the drawings, FIG. 1 shows a process scheme embodiment ofthe present invention that is used to recover NGL products. LNG inletstream 1 is pumped by LNG pump 2 to about 525 psia. LNG stream 3 exitingLNG pump 2 is split into two streams, first feed stream 4 and secondfeed stream 7. First feed stream 4, which is typically the larger of thetwo streams and contains about 95% of LNG stream 3, is preheated infirst heater or first exchanger 5 to about −133.5° F. to provide firsttower feed stream 6. First tower feed stream 6, which is stillsubstantially all liquid, is introduced into fractionation ordistillation tower 10. Second feed stream 7 is heated in first cooler 8to about −132.9° F. to produce second tower feed stream 9, which isintroduced into the fractionation tower 10. In preferred embodiments ofthe present invention, fractionation tower 10 produces a tower overheadstream 14 and a tower bottoms stream 12. Tower overhead stream 14contains the more volatile fraction containing a substantial amount ofmethane contained within LNG stream 3. Tower bottoms stream 12 containsthe less volatile fraction containing the heavy hydrocarbon component.In a preferred embodiment, bottom liquid stream 11 is sent to reboiler13 to produce tower bottoms stream 12. Vapor stream from tower reboiler13 returns to fractionation tower 10. In this embodiment, the reboilercan be used maintain methane content in a NGL product stream 12.Alternate embodiments include substituting other heat sources known inthe art to the bottom of the fractionation column 10.

Tower overhead stream 14 is cooled to about −128.5° F. in first cooler8. This partially condenses at least a portion of the tower overheadstream 14 to produce a partially condensed tower overhead stream 15.Partially condensed tower overhead stream 15 is sent to refluxaccumulator or separator 16. Partially condensed tower overhead stream15 is separated into a lean vapor stream 17 and a tower reflux stream18. Tower reflux stream 18 is sent to fractionation tower 10 preferablyat a top tower feed location. Tower reflux stream 18 can be cooled priorto sending tower reflux stream 18 to fractionation tower 10. Towerreflux stream 18 is sent to fractionation tower 10 preferably by pumpingwith reflux pump 19.

Lean vapor or residue stream 17 is cooled to about −134.5° F. andsubstantially completely condensed in first exchanger 5 therebyproducing lean LNG stream 21. Lean LNG stream 21 is then pumpedpreferably by pump 22 to elevate its pressure to about 1422 psia. Highpressure lean LNG stream 23 leaving the pump is a product stream thatcan be sent for vaporization and/or energy recovery.

The process shown in FIG. 1 can be used when the heavy hydrocarboncomponents include C2 components, C3 components, and heavier components,i.e., ethane recovery. The process shown in FIG. 1 can also be used whenthe heavy hydrocarbon components include C3 components, and heaviercomponents, i.e., propane recovery.

In embodiments of the present invention, the step of cooling at least aportion of tower overhead stream 14 includes cooling at least a portionof tower overhead stream 14 by heat exchange contact with at least aportion of the second feed stream 7 thereby providing at least a portionof duty for the step of preheating the at least a portion of the secondfeed stream. Heat exchange contact between second feed stream 7 andtower overhead stream 14 can be performed in first cooler 8. Inalternate embodiments, such as demonstrated in FIG. 5, In embodiments ofthe present invention, the step of cooling lean vapor stream 17 includescooling lean vapor stream 17 by heat exchange contact with at least aportion of first feed stream 4 in first exchanger 5 thereby providing atleast a portion of the duty for the step of preheating at least aportion of first feed stream 4.

In another embodiment of the present invention, such as is demonstratedin FIG. 7, the embodiments described above in reference to FIG. 1further include the step of compressing the lean vapor stream 17 priorto the step of cooling the lean vapor stream.

In embodiments of the present invention, the tower reflux stream 18 iscooled to create cooled tower reflux stream 26, which is then suppliedto the fractionation tower. The cooled tower reflux stream 26 ispreferably supplied to the fractionation tower at a top feed position.The tower reflux stream 18 is cooled by heat exchange contact with atleast a portion of second feed stream 7 thereby providing at least aportion of the duty for the step of preheating at least a portion ofsecond feed stream 7. In an alternate embodiment, the tower refluxstream 18 is cooled by heat exchange contact with at least a portion ofthe first feed stream 4 thereby providing at least a portion of the dutyfor the step of preheating at least a portion of the first feed stream4.

In certain embodiments, the step of preheating at least a portion of thefirst feed stream 4 to create first tower feed stream 6 advantageouslyincludes supplying the first tower feed stream 6 to the fractionationtower 10 as a tower bottom feed stream. As noted previously, variousmeans of providing heat or energy to the bottom of the fractionationtower are encompassed. An alternate embodiment includes providing thefirst tower feed stream at a position in the column lower than theposition where the second tower feed stream enters the fractionationtower 10. This advantageously provides a driving force to the separationin the fractionation tower 10.

As another embodiment of the present invention, FIG. 3 shows a schemethat uses cooled lean tower reflux stream 26 to increase NGL recovery inNGL product stream 12. In this embodiment, it is advantageous thatcooled lean tower reflux stream 26 be subcooled. Rich low pressure LNGstream 1 is pumped in LNG pump 2 to about 525 psia. LNG stream 3 exitingLNG pump 2 is heated in first exchanger 5 to about −133.2° F. to producefirst tower feed stream 6. First tower feed stream 6, which is stillsubstantially all liquid, is introduced into fractionation tower 10preferably as a bottom feed stream.

In this embodiment, tower overhead stream 14 is cooled to about −133.7°F. and is substantially completely condensed in first exchanger 5 toproduce a low pressure lean LNG stream 21. Lean LNG stream 21 exitingexchanger 5 is elevated in pressure preferably by pump 22 to about 1422psia to produce high pressure lean LNG stream 23. High pressure lean LNGstream 23 is then split into lean tower reflux stream 25 and lean LNGproduct stream 48. Lean LNG stream 48 can be sent for vaporizationand/or energy recovery. Lean tower stream 25 is cooled in firstexchanger 5 to about −233° F. and is then sent, preferably as a top feedstream, to fractionation tower 10. The process described above and itsvarious alternate embodiments can be used when the heavy hydrocarboncomponents include C2 components, C3 components, and heavier components,i.e., ethane recovery. Similarly, the process described above and itsvarious alternate embodiments can also be used when the heavyhydrocarbon components include C3 components, and heavier components,i.e., propane recovery.

In one embodiment shown in FIG. 3, the step of cooling at least aportion of the tower overhead stream 14 includes cooling the portion ofthe tower overhead stream by heat exchange contact with at least aportion of LNG stream 3 thereby providing at least a portion of the dutyfor the step of preheating the LNG stream 3. Alternately or in addition,lean tower reflux stream 25 is cooled by heat exchange contact with LNGstream 3 thereby providing at least a portion of the duty for the stepof preheating the LNG stream 3. In one embodiment, the first tower feedstream 6 is introduced to the fractionation tower below cooled leantower reflux stream 26 to provide additional driving force due toenthalpy differences. The cooled lean tower reflux stream 26 ispreferably supplied to the fractionation tower as a tower top feedstream.

In certain instances, first exchanger 5 can act as a critical pathelement possibly limiting recoveries due to the need to produce acompletely liquid lean LNG stream from the exchanger that can be pumped.Attempts to increase recoveries can result in exchanger pinch on firstexchanger 5 and/or possibly result in a two phase lean LNG stream at theoutlet of first exchanger 5, which would then require furtherprocessing. Regarding FIG. 5, lean gas compressor 27 is introduced toavoid such a pinch and to avoid a two phase stream at the firstexchanger 5 outlet. The bubble point of the lean residue gas streamleaving the tower is increased in this manner. Tower overhead stream 14can be compressed, and then condensed at a higher temperature.

As another embodiment of the present invention, FIG. 5 shows anotherscheme that can be used to separate LNG stream 3 into the more volatilegas fraction containing a substantial amount of the methane and lightercomponents and a less volatile fraction containing a large portion ofthe heavy hydrocarbon components. In this embodiment, LNG inlet stream 1is pumped by LNG pump 2 to about 475 psia. LNG stream 3 exiting LNG pump2 is split into two streams, first feed stream 4 and second feed stream7 defining a second feed stream enthalpy. First feed stream 4, which isthe larger of the two streams and contains about 82% of LNG stream 3, isheated in first exchanger 5 to about −129° F. to produce first towerfeed stream 6 defining a first tower feed stream enthalpy. First towerfeeds stream, which is still substantially all-liquid, and second feedstream 7 are sent to fractionation tower 10. In a preferred embodiment,second feed stream 7 is introduced to fractionation tower 10 at aposition above the first tower feed stream. In one embodiment, secondfeed stream 7 is introduced as a top feed to the fractionation tower 10.In one embodiment, first tower feed stream 6 is introduced as a bottomfeed to the fractionation tower.

In FIG. 5, tower overhead stream 14 is compressed in lean gas compressor27 to about 534 psia. The partially boosted tower overhead stream 28 isthen cooled to about −131.1° F. and is completely condensed in firstexchanger 5 to produce low-pressure lean LNG stream 21. Low pressurelean LNG stream 21 is then pumped preferably by pump 22 to elevate itspressure to about 1422 psia. Stream 23 leaving pump 22 is the highpressure lean LNG product stream that can be sent for vaporizationand/or energy recovery.

As another embodiment of the present invention, FIG. 6 shows a dual leanreflux scheme that utilizes a side reboiler 31 to increase NGL recoveryfrom LNG stream 1. In addition to the elements shown in FIG. 5, sidereboiler 31 introduced in FIG. 6 advantageously maximizes utilization ofcold streams and also minimizes compressor power requirements. As inother embodiments described herein, rich low pressure LNG stream 1 ispumped by LNG pump 2 to about 485 psia. LNG stream 3 is then split intotwo streams, first feed stream 4 and second feed stream 7. First feedstream 4, which is the larger of the two streams and contains about 92%of LNG stream 3, is heated in first exchanger or heater 5 to about−128.4° F. to produce first tower feed stream 6. First tower feedstream, which is still substantially all liquid, is sent tofractionation tower 10. IN one embodiment first tower feed stream is fedto the fractionation tower as a bottom feed stream. Second feed stream 7is introduced preferably as a middle feed stream to fractionation tower10. Tower overhead stream 14 is compressed in lean gas compressor 27 toabout 543 psia. The partially boosted tower overhead stream 28 is thencooled in side reboiler 31 to about −124.1° F.

To provide a portion of the reboiling energy for fractionation tower 10,side reboiler 31 is utilized in this embodiment. Cold tower liquidstream or tower side reboiler stream 29 is heated in side reboiler 31and returned to fractionation tower 10 as stream 30. Partially cooledstream 32 is then further cooled to about −130.7° F. and completelycondensed in first exchanger 5 to produce low pressure lean LNG stream21. Although two separate exchangers are shown to provide cooling fortower overhead stream 28, a single exchanger can be used. Low pressurelean LNG stream 21 is then pumped, preferably by pump 22, to elevate itspressure to about 1422 psia to produce high pressure lean LNG stream 23.Lean LNG stream 23 is then split into a lean tower reflux stream 25 andlean LNG product stream 48. Lean LNG product stream 48 is sent forvaporization and/or energy recovery. Lean tower reflux stream 25 is thencooled in first exchanger 5 to about −232° F. and is then sent tofractionation tower 10, preferably as top feed stream 26.

In this embodiment, the compressed tower overhead stream 28 can cooledby heat exchange with at least a portion of the first feed stream 4thereby providing at least a portion of the duty for the step ofpreheating at least a portion of the first feed stream 4.

FIG. 6 demonstrates an embodiment including splitting the high pressurelean LNG stream 23 to create a lean tower reflux stream 25 and a leanLNG product stream 48. The lean tower reflux stream 25 is then cooled toproduce a cooled lean tower reflux stream 26, which is fed to thefractionation tower 10.

J The embodiments of the invention described herein are applicable whenthe heavy hydrocarbon components include C2 components, C3 components,and heavier components, i.e., ethane recovery. The embodiments describedherein are applicable also when the heavy hydrocarbon components includeC3 components, and heavier components, i.e., propane recovery. 10050]Higher recoveries are made possible by increasing amount of flow infirst tower feed stream 6, which is cold and rich in the embodimentsshown in FIG. 5 and FIG. 6. Recoveries are also enhanced by increase inamount of cooled lean tower reflux stream 26 that is returned tofractionation tower 10. Increasing the discharge pressure of lean gascompressor 27 eliminates exchanger pinch in first exchanger 5 and avoidstwo phase LNG stream from reaching pump 22. The tower pressure can alsobe lowered to increase recovery, at the cost of higher compressionpower. This scheme is able to give high ethane recovery with very highpropane recovery. Comparing results through process simulation withprior art, it can be seen that for a modest increase in power, theprocess in FIG. 6 recovers more ethane. In addition, the recovery ofpropane is also increased. Although the simulations have been carriedout for C2+ (ethane, ethylene, propane, propylene and heavierhydrocarbons) component recovery, the same process can be used for C3+(propane, propylene and heavier hydrocarbons) component recovery.

As yet another embodiment of the present invention, FIG. 7 shows ascheme that uses first cooler 8 as an overhead condenser along with alean gas compressor 27 to recover NGL from rich low pressure LNG 1. Inthis embodiment, rich low pressure LNG 1 is pumped by LNG pump 2 toabout 525 psia. LNG stream 3 is then split into two streams, first feedstream 4 and second feed stream 7. First feed stream 4, which is thelarger of the two streams and contains about 61% of LNG stream 3, isheated in first exchanger 5 to about −126.8° F. to produce first towerfeed stream 6. First tower feed stream 6, which is substantially allliquid, is introduced to fractionation tower 10, preferably as a bottomfeed stream. Second feed stream 7 is heated in first cooler 8 to about−10° F. to produce second tower feed stream 9. Second tower feed stream9 is sent, preferably at a position lower than first tower feed stream7, to fractionation tower 10. In one embodiment, second tower feedstream 9 is a bottom feed.

Tower overhead stream 14 is cooled to about −133.3° F. in first cooler 8thereby partially condensing tower overhead stream. Partially condensedtower overhead stream 15 is sent to reflux accumulator or separator 16.Partially condensed tower overhead stream 15 is then separated into leanvapor stream 17 and tower reflux stream 18. Tower reflux stream 18 ispumped by pump 19 and is sent to fractionation tower 10, preferably as atop feed stream. Lean vapor or residue stream 17 is compressed in leangas compressor 27 to about 596 psia to produce partially boostedcompressed overhead stream 28. Partially boosted compressed overheadstream 28 is cooled, preferably in side reboiler 31 (see FIG. 6), toabout −121.5° F. Alternately, as shown in FIG. 7, partially boostedcompressed overhead stream 28 can be cooled in first cooler 8 to producepartially cooled stream 32. Cold tower side reboiler stream 29 exchangesheat in first cooler 8 and returned to fractionation tower 10 as returnstream 30. Partially cooled stream 32 is then further cooled to about−125.3° F. and completely condensed in first exchanger 5 to produce lowpressure lean LNG stream 21. Lean LNG stream 21 is then pumpedpreferably by pump 22 to elevate its pressure to about 1422 psia toproduce lean LNG product stream 23. Stream 23 is the high pressure leanLNG product that is sent for vaporization and/or energy recovery.

Tower reflux stream 18 can be subcooled to enhance recovery. Thissubcooling can take place in first exchanger 5.

FIG. 8 illustrates another embodiment for recovery of NGL products froma rich lower pressure LNG stream. In this embodiment, overhead expander33 is used to generate reflux for fractionation tower 10 to recover NGL.Rich low pressure LNG stream 1 is pumped in LNG pump 2 to about 550psia. LNG stream 3 exiting pump 2 is heated in first exchanger 5 toabout −125.5° F. LNG stream 3, which is still all liquid, is introducedto fractionation tower 10. In one preferred embodiment, LNG stream 3 isintroduced to fractionation tower 10 as a bottom feed stream.

At least a portion of tower overhead stream 14 is expanded in expander33 to about 365 psia so that tower overhead stream 14 is at leastpartially condensed thereby producing partially condensed low pressurevapor stream 35. Partially condensed low pressure vapor stream 35 isthen sent to reflux accumulator or separator 16 where the stream isseparated into lean vapor stream 17 and tower reflux stream 18. Towerreflux stream 18 is pumped by pump 19. Tower reflux stream 18 is thensubcooled in first exchanger 5 to about −232° F. to produce lean towerreflux stream 26. Lean tower reflux stream 26 is then sent tofractionation tower 10, preferably as a top feed stream. Lean vaporstream 17 is boosted in pressure booster compressor 34, which is drivenoff or powered by the power of expander 33, and then further compressedin lean compressor 27 to about 526 psia. Compressed and warm residue gasstream 28 is cooled in side reboiler 31 to about −115.8° F. to producepartially cooled stream 32. Cold tower side reboiler stream 29 is heatedin side reboiler 31 and returned to fractionation tower 10 as returnstream 30. Partially cooled stream 32 is then further cooled to about−132.6° F. and completely condensed in first exchanger 5 to produce leanLNG stream 21. Lean LNG stream 21 is pumped by pump 22 to elevate itspressure to about 1422 psia. Lean LNG product stream 23 is the highpressure lean LNG product that is sent for vaporization and/or energyrecovery.

FIG. 9 illustrates yet another embodiment that is used to separate rich,low pressure LNG stream 1 by utilizing a subcooled lean LNG stream toincrease NGL recovery and lean gas compressor 27 for a portion of asecond lean vapor stream 17. In this embodiment, rich low pressure LNGstream 1 is pumped, preferably by pump 2, to about 535 psia. LNG stream3 exiting pump 2 is then heated in first exchanger 5 to about −133.4° F.to produce first tower feed stream 6. First tower feed stream 6, whichis still all liquid, is introduced to fractionation tower 10, preferablyas a bottom tower feed stream.

Tower overhead stream 14 is cooled to about −133° F. and partiallycondensed in first exchanger 5 to produce two phase stream or partiallycondensed tower overhead stream 15. Two phase stream 15 is sent tosuction scrubber or second separator 38 that separates two phase stream15 into second lean vapor stream 17 and lean LNG stream 21, which isliquid.

Lean LNG stream 21 is elevated in pressure, preferably by pump 22, to1422 psia to produce a high pressure lean LNG stream 23. High pressurelean LNG stream 23 is then split into lean tower reflux stream 25 andlean LNG product stream 48. Lean tower reflux stream 25 is cooled infirst exchanger 5 to about −232° F. and is then sent, preferably as leantower reflux stream 26 as a top feed to fractionation tower 10.

Second lean vapor stream 17 is compressed in lean gas compressor 27 toproduce a compressed second lean vapor stream 37. Compressed second leanvapor stream 37 is then combined with lean LNG product stream 48 toproduce the lean LNG product that is sent for vaporization and/or energyrecovery. If high pressure lean LNG stream 23 is sent for energyrecovery, then second lean vapor stream 17 can be cooled in sidereboilers that can be added to fractionation tower 10, similar to thatshown in FIG. 6. With the additional cooling for second lean vaporstream 17, energy recovery will be more efficient due to the streambeing colder.

COMPARISON OF PRIOR ART PROCESSES AND EMBODIMENTS OF THE PRESENTINVENTION

TABLE II POWER Tower C2 C3 Pump Compressor Total UA Power LP LNG FIG.RECOVERY % RECOVERY % NGL BPD hp hp hp Btu/hr-F Psia ° F. Hp/gpm 1 87.6398.48 41271 8047 8047 4.44E+06 510 −134.5 6.69 2 88.87 97.73 41520 80558055 4.25E+06 510 −133.7 6.65 3 88.57 99.32 41635 8304 8304 4.54E+06 510−133.7 6.84 4 88.05 97.68 41282 7528 2195 9723 5.43E+06 435 −137 8.08 592.48 98.41 42658 7789 2195 9984 5.48E+06 460 −131.1 8.02 6 94.74 99.6243464 8279 2195 10474 5.93E+06 470 −130.7 8.26 7 98 99.99 44457 82572195 10452 5.30E+06 510 −125.3 8.06 8 96.83 100 44116 8161 2195 103566.98E+06 535 −132.6 8.05 9 96.03 99.99 43882 8097 2186 10283 4.73E+06520 −133 8.03

Table II provides a side-by-side comparison of prior art processes anddescribed embodiment of the present invention. As can be seen in TableII, the prior art process shown in FIG. 4 has the lowest recovery ratesfor both C2 and C3 recoveries when compared to the process embodimentsdescribed herein.

The invention also encompasses the apparatus necessary for each processembodiment. A preferred embodiment of the apparatus for separating aliquefied natural gas (LNG) stream into a more volatile gas fractioncontaining a substantial amount of the methane and lighter componentsand a less volatile fraction containing a large portion of the heavyhydrocarbon components includes means for splitting LNG stream 3 into afirst feed stream 4 and a second feed stream 7. Various means are knownin the art from a simple T in a line to more complex vessels. Firstexchanger 5 is operable to preheat first feed stream 4 thereby creatingfirst tower feed stream 6. First tower feed stream is fed intofractionation tower 10. Fractionation tower 10 produces tower overheadstream 14 including the more volatile fraction containing thesubstantial amount of the methane and lighter components and a towerbottoms stream 12 including the less volatile fraction containing theheavy hydrocarbon components. At least a portion of tower overheadstream 14 is cooled by first cooler 8, which is operable to cool andpartially condense at least a portion of the tower overhead stream 14 toproduce a partially condensed tower overhead stream 15. In a preferredembodiment, first cooler 8 is operable to allow for heat exchangebetween the portion of the tower overhead stream 14 and the second feedstream 7. Separator 16 is provided which is operable to separate thepartially condensed overhead tower stream 15 into a lean vapor stream 17and a tower reflux stream 18. Tower reflux stream 18 returns to and isfed to the fractionation tower 10. In a preferred embodiment, firstexchanger 5 is operable to allow for heat exchange between the leanvapor stream 17 and the first feed stream 4, the lean vapor stream 17being cooled in the first exchanger 5 to provide a lean LNG stream 21.Pump 22 is operable for pumping the lean LNG stream 21 to a higherpressure to produce high pressure LNG stream 23.

In another embodiment, first compressor 27 is operable to receive leanvapor stream 17 and boost the pressure to produce compressed overheadstream 28. Another embodiment includes side reboiler for supplying atleast a portion of reboiling requirements for the fractionation tower.

While the invention has been shown or described in only some of itsforms, it should be apparent to those skilled in the art that it is notso limited, but is susceptible to various changes without departing fromthe scope of the invention.

1. A process for separating a liquefied natural gas (LNG) streamcontaining methane and lighter components and heavy hydrocarboncomponents into a more volatile gas fraction containing a substantialamount of the methane and lighter components and a less volatilefraction containing a large portion of the heavy hydrocarbon components,the process comprising the steps of: (a) splitting the LNG stream into afirst feed stream and a second feed stream; (b) preheating at least aportion of the first feed stream to provide first tower feed stream andsupplying the first tower feed stream to a fractionation tower toproduce a tower overhead stream including the more volatile fractioncontaining the substantial amount of the methane and lighter componentsand a tower bottoms stream including the less volatile fractioncontaining the heavy hydrocarbon components; (c) preheating at least aportion of the second feed stream to provide second tower feed streamand supplying second tower feed stream to the fractionation tower; (d)cooling and partially condensing at least a portion of the toweroverhead stream to produce a partially condensed tower overhead stream;(e) separating the partially condensed tower overhead stream into a leanvapor stream and a tower reflux stream, and supplying the tower refluxstream to the fractionation tower; (f) cooling the lean vapor streamsuch that the lean vapor stream is substantially condensed therebyproducing a lean LNG stream; and (g) pumping the lean LNG stream to ahigh pressure lean LNG stream.
 2. The process according to claim 1,wherein the heavy hydrocarbon components include C2 components, C3components and heavier components.
 3. The process according to claim 1,wherein the heavy hydrocarbon components include C3 components andheavier components.
 4. The process according to claim 1, furthercomprising the step of compressing the lean vapor stream prior to thestep of cooling the lean vapor stream.
 5. The process according to claim1, wherein the step of cooling at least a portion of the tower overheadstream includes cooling at least a portion of the tower overhead streamby heat exchange contact with the at least a portion of the second feedstream thereby providing at least a portion of duty for the step ofpreheating the at least a portion of the second feed stream.
 6. Theprocess according to claim 1, wherein the step of preheating at least aportion of the first feed stream and supplying the first tower feedstream to the fractionation tower includes supplying the first towerfeed stream to the fractionation tower as a tower bottom feed stream. 7.The process according to claim 1, wherein the step of preheating atleast a portion of the first feed stream and supplying the first towerfeed stream to the fractionation tower includes supplying the firsttower feed stream to the fractionation tower at a position in thefractionation tower lower than the position where the second tower feedstream enters the fractionation tower.
 8. The process according to claim1, wherein the step of separating the partially condensed tower overheadstream into the lean vapor stream and the tower reflux stream includessupplying the tower reflux stream as a top tower feed stream to thefractionation tower.
 9. The process according to claim 1, wherein thestep of cooling the lean vapor stream includes cooling the lean vaporstream by heat exchange contact with at least a portion of the firstfeed stream thereby providing at least a portion of the duty for thestep of preheating at least a portion of the first feed stream.
 10. Theprocess according to claim 1, wherein the process further includes thestep of cooling the tower reflux stream to create a cooled tower refluxstream and supplying the cooled tower reflux stream to the fractionationtower.
 11. The process according to claim 10, wherein the step ofsending the cooled tower reflux stream to the fractionation towerincludes sending the cooled tower reflux stream as a top feed stream tothe fractionation tower.
 12. The process according to claim 10, whereinthe step of cooling the tower reflux stream includes cooling the towerreflux stream by heat exchange contact with at least a portion of thefirst feed stream.
 13. A process for separating a liquefied natural gas(LNG) stream containing methane and lighter components and heavyhydrocarbon components into a more volatile gas fraction containing asubstantial amount of the methane and lighter components and a lessvolatile fraction containing a large portion of the heavy hydrocarboncomponents, the process comprising the steps of: preheating at least aportion of the LNG stream and supplying the at least a portion of theLNG stream to a fractionation tower as a first tower feed stream toproduce a tower overhead stream including the more volatile fractioncontaining the substantial amount of the methane and lighter componentsand a tower bottoms stream including the less volatile fractioncontaining the heavy hydrocarbon components; expanding at least aportion of the tower overhead stream to a lower pressure such that theat least a portion of the tower overhead stream is partially condensedto produce a partially condensed low pressure vapor stream; separatingthe partially condensed low pressure vapor stream into a lean vaporstream and a tower reflux stream; compressing the lean vapor stream;cooling the lean vapor stream to create a lean LNG stream; cooling thetower reflux stream thereby producing a cooled lean tower reflux stream;and pumping the lean LNG stream to a high pressure lean LNG stream. 14.The process according to claim 13, where the heavy hydrocarboncomponents include C2 components, C3 components and heavier components.15. The process according to claim 13, where the heavy hydrocarboncomponents include C3 components and heavier components.
 16. The processaccording to claim 13, whereby the tower reflux stream is cooled by heatexchange contact with at least a portion of the LNG stream prior to thestep of supplying the tower reflux stream 18 to the fractionation tower.17. The process according to claim 13, the step of supplying the towerreflux stream to the fractionation tower includes supplying the towerreflux stream to the fractionation tower as a top feed stream.
 18. Theprocess according to claim 13, wherein the step of cooling the leanvapor stream includes cooling the lean vapor stream by heat exchangecontact with at least a tower side reboiler stream thereby providingreboiling duty to the fractionation tower.
 19. The process according toclaim 13, wherein the step of cooling the lean vapor stream includescooling the lean vapor stream by heat exchange with at least a portionof the LNG stream thereby providing at least a portion of duty for thestep of preheating the at least a portion of the LNG stream
 3. 20. Theprocess according to claim 13, wherein the step of supplying the atleast a portion of the LNG stream to the fractionation tower includessupplying the at least a portion of the LNG stream to the fractionationtower as a tower bottom feed stream.
 21. An apparatus for separating aliquefied natural gas (LNG) stream containing methane and lightercomponents and heavy hydrocarbon components into a more volatile gasfraction containing a substantial amount of the methane and lightercomponents and a less volatile fraction containing a large portion ofthe heavy hydrocarbon components, the apparatus comprising: means forsplitting the LNG stream into a first feed stream and a second feedstream; a first exchanger operable to preheat the first feed stream toprovide a first tower feed stream; a fractionation tower for receivingthe first tower feed stream to produce a tower overhead stream includingthe more volatile fraction containing the substantial amount of themethane and lighter components and a tower bottoms stream including theless volatile fraction containing the heavy hydrocarbon components; afirst cooler operable to cool and partially condense at least a portionof the tower overhead stream to produce a partially condensed toweroverhead stream, the first cooler operable to allow for heat exchangebetween the portion of the tower overhead stream and the second feedstream; a separator operable to separate the partially condensedoverhead tower stream into a lean vapor stream and a tower refluxstream, the tower reflux stream operable to return to the fractionationtower, the first exchanger being operable to allow for heat exchangebetween the lean vapor stream and the first feed stream, the lean vaporstream being cooled in the first exchanger to provide a lean LNG stream;and a pump operable for pumping the lean LNG stream to a high pressureLNG stream.
 22. The apparatus of claim 21, further including a firstcompressor for compressing the lean vapor stream.
 23. The apparatus ofclaim 21, further including a side reboiler for supplying at least aportion of reboiling requirements for the fractionation tower.